Process for removing silica from water



April 11, 1950 E. J. ROBERTS 2,503,769

PROCESS FOR REMOVING SILICA mou WATER Original Fiied April 14, 1945 A00rllo r230 I0 FEED II HF 22 NdOH WATER B I '5.

2 MIXANG 24 REACTION TANK F /G.-

BED OF 1 ANION A EXCHANGE I MATERIAL F /6 3 INVENTOR ELLIOTT J. ROBERTSATTORNEY Patented 1'1, 1950 UNITED STAT S; PATENT e es I mouse mum J.Roberts, westn rt. com, assignor to The Don Company, Stamford, Com, acorporation oiDelaware Original application April 14, 1945, Serial No.-588,388. Divided and this application June 2,

1947, Serial No. 751,682

1 Claim. (01. tic-'24) high .pressure operating conditions boiler tubefailures have often been traced to siliceous deposits and trouble isalso encountered from these deposits on turbines and in superheaters.

All sources of natural waters contain some dissolved silica and surfacewaters, a usual occurrence being about -15 P. P. M. S102, and inaddition usually contain suspended silica. The removal of suspendedsilica can generally be obtained by coagulation and filtration whilechemical processes are necessary for removal of the colloidal anddissolved silica. Several chemical processes have been developed forthis purpose, but nearly all fail to remove the silica to -a low enoughtolerance and many processes increase the salt content of the water. Thesilica tolerance or maximum amount of silica allowed in the boiler feedwaters have not been definitely established, but as complete as possiblea removal of silica is desirable, especially under high pressure boileroperating conditions, because of the greater tendency of the S10: toform highly insoluble compounds at the higher temperatures and higherpressures. Indeed, siilca compounds formed under these conditions maynot entirely yield even .X Hi0 NazSiO:

N8OH.+ 510, x1110 which merely changes the silica to a. form lessscale-producing, but still presenting the problem ofreducing thesereaction products to a. minimum concentration so as to avoid theirpossible corrosive effect within the boiler system.-

Another example of conventional methods of silica removal is what maybriefly be called the Magnesia Method. Magnesia (MgO) or magne- Acomblnationmethod provides for removing first one portion of the S10:bymagnesia treatment, and converting the remainder by NaOH- treatment.

While feed water free of Sid: and other solutes or salts is attainableby using the condensate of a low pressure boiler as feed water fora'high pressureboiler, this invention'has for its object to devise achemical method for effecting economical- .ly the substantially completeremoval of $102 the treatment of the water in two sequential exchangestages, namelyflrst with a cation exchange material and then with ananion exchange material, whereby the solutes which are susceptible tosuch treatment are chemically replaced with the molar equivalent of purewater. An understanding of the operation of this sequential ex- I changeprinciple is desirable in connection with the present invention, and itis therefore more fully explained as follows:

The cation exchanger in that instance is saturated with H-ions andtherefore also called an H-ion exchanger. It releases H-ions into thewater in exchange for the molar equivalents of cations oi. the solutes.To the extent of that ex-' change the corresponding acid is formed inthe water passing through a bed of H-ion exchange material. The waterthus acidified is then passed I through a bed of anion exchange materialcapable of neutralizing the acid in the sense that it releases OH-ionsin exchange for the anions of the acid, forming pure H20, or else in thesense that the acid molecule as such is adsorbed bythe exchanger. Theanion exchange .material is therefore also known as an acid adsorbingmate-v rial. In due course of such operation the exslum hydroxide (MgOH)added to the water binds the S102 and precipitates it in some flocculentform removable by filtration; yet this introduces andleaves in the wateran excess of the conditioning chemical which in turn increases the waterhardness and eventually forms scales in combination with $102. Moreover,an undue excess of conditioning chemical would be required to remove thesilica to the extent desired.

change materials lose their exchange capacity which can be restored byregeneration, that is by contact with a suitable regenerant solutionwhich in the case of the H-ion exchanger is a suitable mineral acid suchas H2804 or H01, andiin the case of the anion exchange material analkali such as NaCOz, of suitable concentration.

While the sequential cation and anion exchange treatment will serve toabstract from the water solutes sufllciently strongly ionized, there areinstances or treatment problems involving the use of only the one or theother of these exchange stages. For instance, a water or liquid may besubjected to H-ion exchange only, and the resulting liquid beneutralized in some manner other than by anion exchange, or else a wateror liquid already acid may be de-acidified by being contacted with theanion exchange or acid-adsorption material. a

This invention proposes to effect a substantially complete removal ofthe silica ($102) by subjecting the water to an auxiliary reactionconverting the silica in the water into a suitable acid, and thenremoving that acid from the water by contact with an acid-adsorbing oranion exchange material.

When water to be treated passes downwardly through a bed of exchangematerial, the exhaustion of the bed progresses downwardly from end toend of the bed; that is, the exhausted upper portion of the bed keeps ongrowing downwardly as the unexhausted portion below it diminishes, untilexhaustion has reached the bottom of the bed, at which time regenerationis required. The substantial exhaustion of the exchange capacity of thebed as a whole is indicated b what is known as the breakthrough; that isthe appearance of those ions in the eflluent which the exchange materialis normally expected to remove. If a regenerant solution is passedthrough the bed, then regeneration proceeds in a similar progressivefashion, namely from one end of the bed to the other. It can be saidthat a certain exchange material has a certain-inherent exchangecapacity as well as inherent regeneration requirements underpredetermined operating conditions.

Cation as well as anion exchange materials adapted to function in themanner above indicated, are exemplified by a group of materials nowknown as organolites because they are of an organic, that is syntheticresinous nature, as distinguished from earlier cation exchangematerials, the so-called zeolites, which are of inorganic nature. a

This invention proposes to subject the water to an auxiliary treatmentstep or reaction whereby the silica (SiO2) is converted into a suitableacid, namely one that is removable from the water by ion exchangetreatment such as can be effected by means of the aforementionedtreatment with a regenerable anion exchange or acid-adsorbing material.

In order to provide for this auxiliary reaction a quantity of fluoridemay be supplied in the form of NaF although the Na+ must be replacedwith H+ as by way of cation exchange, that is to say by contact with anacid-regenerated cation exchange material.

According to one feature, the raw water is first subjected tode-mineralization treatment by sequential contact with cationand anionexchange materials, which treatment is followed by the silica removaltreatment which is based on the concept that the silica must beconverted into an acid which in tuzn can be removed by the anionexchange material.

This invention proposes an auxiliary treatment which comprises reactingthe silica (SiOa) with hydrofluoric acid (HF) to the end of producinghydrofiuosilicic acid (HzSiFe) which in turn is abstracted from thewater by treatment with the anion exchange material.

In an embodiment of this invention, a zone or band of HF held by theanion exchanger is allower or caused to form in the exchange bed, andthis zone or band by way of the ion exchange phenomena taking place isin effect caused to progress or shift through the bed ahead of the zoneof exhaustion that develops in the bed; that is as the exchange bedbecomes progressively exhausted by the acid reaction product (HzSiFs)being taken up, there is maintained in advance of that exhausted portionthe band or zone of HF.

To embody this invention, the conditioning of the anion exchange bedfollowing its regeneration is effected with alkali, so that it willsubsequently and immediately function at highest efficiency with regardto SiOz removal. The conditioning step includes passing through the beda sumcient quantity of HF solution to establish a desirable HF-zone atthe influent end of the bed prior to starting the passage of waterthrough the bed for silica removal.

To embody this invention, a bed of anion exchange material serving toeffectuate the auxiliary reaction as well as the removal of theresulting reaction product or acid (HzSiFc) is operated in a mannerwhereby the fluoride (HF) breakthrough and the subsequent silicabreakthrough serve as criteria indicating the degree of exhaustion ofthe anion exchange bed.

To embody this invention, the water from which the silica is to beremoved is passed through a pair of anion exchange beds operating inseries, whereby the first bed can be substantially fully exhausted witha minimum loss of HF. The complete exhaustion of the first bed ispossible according to this mode of operation since the HF- zone or bandreaching the end of the first bed is eventually further displacedtherefrom and without loss transferred to the next fresh bed while watercontinues passing through these beds. The transfer of the HF-zone is dueto certain affinities of the solutions involved with respect to theexchange material, as will be more fully explained.

An embodiment of this invention provides that to the water having beende-ionized by sequential anion and cation exchange treatment, thereshall be added substantially only the theoretical amount of HF neededfor reaction with the SiOz of the water. a

To embody this invention there are employed modes of regenerating theanion exchange bed exhausted under these conditions with silicacompound, in such a manner that the silica is substantially completelyremoved from the bed in spite of the tendency of silica to beprecipitated on-the exchange material; that is, this feature relates tomodes of regenerating the bed in a manner whereby the silica is removedfrom the bed in a soluble form as effluent.

Therefore, the anion exchange bed having been saturated or exhaustedwith HaSiFs is regenerated with an alkali regenerant such as NaOH at anunusually high dilution.

The invention possesses other objects and features of advantage, some ofwhich with the foregoing will be set forth in the following description.In the followin description and in the claim, parts will be identifiedby specific names for convenience, but they are intended to be asgeneric in their application to similar parts as the art will permit. Inthe accompanying drawings there has been illustrated the best embodimentof the invention known to me, but such embodiment is to be regarded astypical only of many possible embodiments, and the invention is not tobe limited thereto.

The novel features considered characteristic of the appended claim. Theinvention itself. however. both as to its organization and its method ofoperation. together with additional objects and advantages thereof, willbest be understood from the following description of a specificembodiment when read in connection with the accompanying drawings inwhich Fig. 1 is a flowsheet diagram for the removal of silica accordingto the Non-Cyclic Process.

Fig. 2 is a schematic showing of the anion exchange bed to illustratethe function of the HF band.

Fig. 3 is a schematic showing of the anion exchange bed in .variousstages of exhaustion. and illustrating the progress of the HF bandthrough the bed and its final complete displacement.

Fig. 4 is a schematic view, of a pair of anion exchange beds operatingin series, in the process of being exhausted, to illustrate the transferof the HF band from a first to a second bed.

The flowsheet diagram of Fig. 1 of the Non- Cyclic Process provides afeedwater tank It;

that is, a tank for storing the water from which the silica. is to beremoved, provided with a feedwater supply It. The feedwater to besubjected to the silica removal treatment of this invention.

should be substantially free from dissolved salts. Therefore it isdesirable to have the ionized solutes substantially removed from thewater by a preceding de-ionization treatment, whereby the non-' ionizedsilica is left in the water to be separately removed as by this process.For the sake of simplicity it will be assumed for this flowsheet thatdeionized water having the silica left I in it is contained in tank iii.A tank H provides for the storage of the initial conditioning reagent,namely HF solution of a suitable concentration, for admixture to thefeedwater. The tank I I has a connection II for supplying HF thereto. Amixing or reaction tank I! is provided for such conditioning. Flowconnections or conduits i3 and I4 lead from the tanks l0 and IIrespectively to the mixing and reaction tank It, and are provided withcontrol valves 15 and i6 respectively. A flow connection ll providedwith control valve It leads from the reaction tank i2 to a tank or celll9 containing a bed of granular anion exchange ma.- terial 20, alsotermed acid-adsorbing material, for instance a material of the organicor synthetic resinous kind such as is exemplified by the anion exchangematerial designated as I-R 4 by the Resinous Products Company ofPhiladelphia. An example of a known organic anion exchange material ofthis character is a soluble resin-like product obtained by the reactionof formaldehyde with an aromatic amine. The tank is will herein betermed the exchanger tank or cell. An eiiluent conduit 2|, having acontrol valve 22, leads from the bottom of the exchanger tank l9 andforms a. goose neck G presenting a hydraulic column suificiently high tobalance the liquid level in the exchange tank whereby the substantialsubmersion of the exchange bed 20 during operation is insured.

The feedwater from tank i0 and the HF solution from tank II are mixed inthe conditioning or reaction tank [2 where-silica of the water reactswith HF to produce hydrofluosilicic acid as follows:

(1) 6HF+SiO2- H2SiFs+ 2H2O The resulting acid is then removed from thewater by contact with the anion exchange bed 20 through which the wateris passed for instance downwardly. thus producing a substantiallyallica-free eiiluent water leaving the bed through-an eiiluent conduit2|.

A storage ink 23 with a supply therefor indicated at 23' contains NaOHfor regenerating the anion exchange bed 20 after it has been exhaustedwith HaSiFo. a supply connection 24 with control valve 25 leading to theexchange tank it.

While the overall function of the anion bed in HF to the bed afteralkali regeneration before any water is passed through the bed fortreatment. In this way a protective band of HF absorbed on the exchangeris established and if water containing HzSiFs is passed through such, apreconditioned bed, the eiiluent is silica-free'right from the start ofthe water treatment phase. This band progresses through the exchange bedas the bed becomes progressively exhausted. The emcient removal of thesilica is due to the presenceof this band because it prevents directcontact of the HiSiFs in the water with the unexhausted (alkaliregenerated) portion of the bed. In other words the water always passesthrough an HF-zone before it contacts the alkali regenerated portion ofthe bed. The HaSiFs displaces HF from the infiuent side of the HF-zoneand the displaced HF is reabsorbed-by the fresh exchange at the eiiluentside of the zone. In this way the aforementioned progress of the HF-bandtakes place. Chemically the function of'this HF-band is that it preventsdecomposition of the HrSiFa with consequent formation of silica andleakage thereof with the treated water, which would take place were theHaSiFe allowed to come in direct contact with the unexhausted portion ofthe bed which is alkaline in reaction and would therefore cause theaforementioned decomposition of the HzSiFs.

Another important aspect of the formation and effect of the HF-band isthat with such an HF- band properly established in the bed. reasonablefluctuations in the HF dosing rate will be equalized as far as theeiiluent is concerned, in that the HF-band decreases or increases inabsorbing these dosing fluctuations. This equalizing eflect enables oneto dose with the theoretical quantity of HF without danger of silicaleakage into the eiiluent even though in practical operation precisedosing rates may not be possible.

This condition can be graphically visualized by reference to the Fig. 2diagram showing the anion exchange-bed N in an intermediate state ofexhustion. A top zone A saturated with HaSiFs represents the exhaustedportion of the bed, while a relatively shallow intermediate zone or bandB is saturated with HF, and a lower or bottom zone C represents theunexhauted portion of the bed.

Since it is desirable that the HF-band be present at the top.of the bedwhen the feedwater is started into the bed such a band or zone may beestablished prior to starting the operation by adding a sufficientamount of HF to the top of the bed, the bed itself having previouslybeen saturated with OH-ions by regeneration with an alkali solution, forinstance NaOH.

The progress of exhaustion of the anion exchange bed, as indicated bythe progress of the HF-band therethrough, is graphically shown in Fig. 3(sub-figures a to 6) representing an eurband D, a zone of exhaustedmaterial E, and a zone of unexhausted material F of the bed. Thesefigures incidentally indicate the bed as being about 50%, 70%, 95%, and100% exhausted, respectively. Consequently the HF-band is shown inconsecutive stages 01' downward progress through the bed. In the Fig. 0;condition the her is unexhausted and an HF-band has been provided at thetop. In the Fig. 2; condition the HF-band has moved approximatelyhalfway down the bed. In the Fig. c condition the EW- bandapproaches thebottom of the bed as the same approaches exhaustion. In the Fig. dcondition a portion of the I-lF-band has been displaced by HzSiFeindicating what is herein called the condition of the HF breakthrough ofthe bed, since the displaced portion of the IFband now appears in theefiluent water. If the exhaustion is still further continued the HF-bandwill be completely displaced, leaving the bed substantially totallyexhausted or saturated with HzSiFo, this being the Fig. 6 conditionwhich is substantially incidental to what is herein termed the silicabreakthrough condition of the bed; that is, as the HF-band disappears,the efiuent water will then show it is substantially the same conditionas the influent water-with a portion of HzSiFs, a portion of S102 and aportion of HF- since the bed will then have become substantiallyinefiective.

In order to preserve the HF contained in the HF-band a pair of anionexchange beds may be operated in series (see Fig. 4). This shows twoanion exchange beds 31 and B2 in series, the feedwater entering at Wflowing downwardly through bed B1, the efliuent water from that bedbeing transferred as along line L to the top of bed B2 and passingthrough the same and leaving the bed as silica-free water indicated atS. The beds B1 and B2 are shown in a condition where the bed comprises aportion P2 of the HF-band and an unexhausted portion U. In this way anHF-band will be established at the top of the second bed at the rate atwhich it is being displaced at the bottom of the first bed. In thismanner the first bed can be completely exhausted past the silicabreakthrough condition, since the HF-band being displaced at the bottomis intercepted and reestablished at the top of the second bed. Thiscondition of transferring the exchange function as well as the HF-bandfrom the first bed to the second bed is indicated in Fig. 4 showing afirst bed B1 and a second bed B2 with a portion oi the HF-band stillleft at the bottom of the first bed and the balance of the HF-bandhaving been established at the top of the second bed. In other words,the HF-band desired to be present at the start of the operation of ananion exchange bed can be automatically obtained by using a series ofbeds if a second regenerated and Washed bed is placed in series with abed being exhausted shortly before the fluoride breakthrough occurs, thefluoride leakage will be cut by the second bed. Hence, if the first bedis exhausted until the silica in the efiluent is nearly identical withthe silica in the ieedwater the HF-band will be completely transferredfrom the bottom of the first to the top of the second bed because thestrong bivalent HzSiFs displaces the weak monovalent HF from the one bedto the next.

The following tabulation represents an eflluent water analysis duringthe exhaustion period of the anion exchange bed indicating numericallywhere the fluoride breakthrough and the silica breakthrough occurrespectively, this being the result of a test run on an anion exchangebed 10 deep and 1" in diameter. There is a rough relationship betweenthe degree of exhaustion shown in the diagram Fig. 3 (sub-figures a toe) which is indicated 'by the marginal reference to these figures in thefollowing Eilluent Analysis tabulation.

Volume of Feed Per Cent Exhausted to SiO: Fluorides through Bed SilicaBreakthrough PH spew" 881mm P.P.M. P. M.

liters 2 s. s 7.2 280, 000 .2 0 Fig. 3a 4 07 7.15 365,000 .2 0 8 13.47.10 380,000 .1 0 12 go wwneud 400,000 0 23 a3 E 3 e85 '1 g q: o 24 40.0sn ge 0.50 a 380,000 .1 0 28 47.0 n g-55 5.20 @E 175,000 .1 0 Fig.3!) 3050.0 ogmiggg 5.00 cg {150,000 .1 .1 32 53.0 4.7 g 100,000 .1 .1 30 00.0figaagso 4.5 1: 80,000 .1 .2 as 63.0 he a4 g 05,000 0 .0 40 07.0 4.3 a41,000 0 1.1 Fig. .10

42 700 3 4.12 28,000 0 1.4 45 75.0 31's: 4.00 20,000 0 2.0 Q '5 83.0h-10 3.70 11,000 0 2.9 gain 02.0 3.50 0,500 0 5.0 am 58 97.0 e I 3.404,800 .2 10.5 8 Fig.3d 00 100.0 1 3.35 4.200 1.0 2220 as;

02 104. 0 3. 20 3, 500 2. 0 24. 20 Fig. s0 64 107.0 3. 20 2, 700 4.0 28.20 00 110. 0 a. 20 2, 100 0. 0 :12. as 114.0 s. 20 1.800 0.0 40. 117. 03. 20 1, 700 12. 0 50. 72 120. 0 s. 20 1, 700 10. 0 55.

B1 has been exhausted to the point where a 79 Alternative modes ofinitial treatment of the portion of the HF band has been displaced fromthe bottom of the bed B1 and appears transferred to the top of the bedB2. The bed B1 in this condition comprises an exhausted portion E, and

raw water in this process are herein to be considered each of whichmodes has its advantages depending upon the character and analysis ofthe raw water; that is the analysis of the solutes a portion P1 of theHF-band, while the bed B2 7 or salts other than the silica.

one mode of initial treatment is that which is indicated above, namelywhere the raw water is first subjected to a. preliminary orde-lonizatlon treatment not shown per se in theilowsheet of Fig. 1. Sucha preliminary treatment comprises passing the water sequentially througha bed of cation exchange material and through v a bed of anion exchangematerial, whereby substantially all inorganic salts or solutes exceptthe silica are abstracted from the .water. The chemical mechanism of thede-salting or deionization treatment is well known per se. Suffice it tosay that *the cation exchange bed, hav- 7 ing been regenerated with asuitable mineral acid such as NazCOa of suitable concentration, and istherefore capable of adsorbing or. abstracting from the water the acidwhich was produced by the cation. It isalso said of the anion exchangebed that itsubstitutes OH-ion forthe anion of the acid which hasresulted from the cation exchange so that as a net result of these twoion exchange phases the molar equivalent of pure water (HOH- or B20) issubstituted. for the salt.

The removal of these salts by this preliminary or de-ionizationtreatment correspondingly reduces the I-IF requirement for conditioningthe water, inasmuch as otherwise some of the HF would react with thesalts instead of with the silica. Also, having'an appreciable Ge.content Ca may react with HF to produce sufficient CaFz which is fairlyinsoluble causing precipitation trouble in the bed which in turnrequires more intense backwashing' for precipitate removal;

thus the preliminary or de-ionization treatment in the conditioningstages of the raw water may be desirable and the expenditures for it warI ranted.

The HF per se is a corrosive acid and may be produced as needed bytreating NaF separately.

by cation exchange substituting H for Na.

When'raw water containing, as it usually does,

. calcium bicarbonate (Ca(HCOa)2) is subjected to treatment in thecation bed in thede-ionization operation, there is produced free C02dissolved in the water as follows:

Ordinarily this remains in the .water when the same is passed throughthe acid adsorbing anion exchange bed.

In straight de-ionization operations this CO2 is often removed byaeration. In the present process a limited amount of C0: in the waterdoes not interfere with the efliciency of the silica removal, but whereexcessive amounts are present and the efilclency ofthe silicaremovalthereby affected, the CO3 should be removed by aeration prior to thedosing of the water with HF.

When the anion exchange bed has become .exhausted by its adsorption ofHaSiFc it must be regenerated with a solution of alkali such as NazCOa,NaHCOa, or NaOH. Usually 5% concentrated solutions of alkali are used toobtain satisfactory and economical regeneration resaturated andprecipitates.

With an excess of NazCO: the resulting NazSiF'o reacts further to form aflocculent (S101) -precipitate and (GNaF) solute, as follows:

a 2NaCOg NatSiF. ----t 510, dNaF 200,

i For NaI-ICOa:

Similarly:

Nmsm. a) zmnco. msir.

and NazSlFc reacting with an excess of NaHCOa as follows: (3G) inn-10o.NmBiF.

For NaOH: (4) 2NaOH+H2SiFe- NazSiFc+2I-I2O and NazSiFa reacting with anexcess of NaOH:

SiO:

. Bio: (4G) 4NaOH NBaSlFd 6N5]? 211:0

Under such conditions I have found that, once the silica hasprecipitated in the bed, it can be rendered soluble and removed only bya treatment of excess strong NaOH solution (10% NaOH) producing solublesodium silicate as follows: I Y

S102 +2NaOH- NazSiOs +H2O However, I have found that the precipitationtrouble as well as the cost of precipitate removal can be avoided byusing the alkali regen'erant, such as NaOH, in relatively high dilution,for in stance on the order of 0.5%. In other words, when the anion bedhas been exhausted with the water containing the -H2SiF6, it isregeneratedwith dilute caustic. The strength of the caustic solutionthat can be used for regeneration is determined by the temperature ofthe surroundings, and this concentration should not be so high that theconcentration of the NazSiFs produced in the regeneration becomes super-Hence, at a temperature 'of 25 C. the NaOH concentration should be onthe order of 0.4% by weight or about 0.1 N. This strength is probablysafe down to a temperature slightly below 20 C. If it were possible tomaintaina temperature of 40 C., the

strength of NaOH could be increased by about 30% because of theincreased solubility of NazSiFe. At C. the concentration would beincreased about to about 0.2 N.

The regeneration with the dilute NaOH solution takes place according tothe following equation in terms of ion exchange:

+ GNBF 21110 400:

(where Y represents the structure of the anion I exchanger); or writtenas a chemical equation:

This equation is actually the result of the operation of ,,Equations 4,4a. and 5. Therefore an additional amount of reagent is consumed in thatthe silica is removed from the bed as NazSiOs instead of as NazSiFs.

Starting with a freshly alkali regenerated anion exchange bed, in orderto efiect the substantially complete removal of the silica from thefeedwater at the outset where the feedwater had been dosed with thetheoretical quantity of HF, I have conditioned the influent end portionof the bed by adding a small quantity of dilute HF solution to the :beduntil a small band of HF was formed at the influent end. The quantity ofHF required to produce a satisfactory band of the acid -I have found tobe about 6 to 10 equivalents of HF per square foot of bed area. Theanion exchange or acid-adsorbing material used was that by the ResinousProducts Company identified as IR-4 although the anion exchange materialnamed Duolite by the Chemical Process Company of San Francisco, Calif.represents about the equivalent in capacity and might be substituted.

As an example, a 10 inch bed of granular anion exchange material in a 1plastic tube was regenerated with caustic and then exhausted downflowwith solution or feedwater containing 60 P. P. M. SiOz as H2SiFs. Thebed adsorbed 131 in. eq. H2SiFs (m. eq.=milligram equivalents) duringexhaustion. After the bed was exhausted it was backwashed to loosen thebed.

The exhausted bed was regenerated upflow with 3.32 liters of 0.1 NNaOI-I. A flow rate of 0.5 gal/sq. ft./min. was used during theregeneration. After regeneration the bed was washed with de-ionizedwater.

In order to be certain that substantially complete conversion ofdissolved silica to HzSiFs in the feedwater would take place, in. eq. ofHF was added to the top of the bed. Then the feedwater containing 60 P.P. M. SiOz as H2SiFs was passed through the bed.

The efliciency of silica removal from the bed with 0.1 N NaOH appearedto depend upon the flow rate used in the regeneration. Based onregeneration effluent solution analysis, it appeared that when aregeneration flow rate of 0.5 gal./sq.ft./min. was used, completeremoval of S102 was obtained with about 4 mols of NaOH per mol of SiOzwhile at 1 gal./sq.ft./min. it required about 5 mols per mol of SiO2 toremove all of the silica from the bed. With the higher flow rate, moreof the NazSiFe is converted to NaF.

The capacity of the anion exchange material (IR-4) for HzSiFs with afeedwater of 60 P. P. M. of silica was about 4.5 m. eq. per dry gram or35,400 m. eq. per cu. ft. to the silica breakthrough. When an excess HFamounting to about 6 equivalents or gm. of HF per square foot of bedarea was added to the top of a 10 inch deep bed, the capacityto thefluoride breakthrough was about 70% of the above figure or 3.2 m. eq.per gram of 112-4. Some fluoride leakage (0.2-1.0 P. P. M.) in theefliuent water was observed shortly before the fluoride breakthroughoccurred.

The exhaustion-or saturation of the bed with HzSiFe proceeds as the HFband moves through the bed in themanner and under conditions describedabove. The net result of the exhaustion of the bed may be represented bythe following equation:

where Y is the acid absorbing radical of the anion exchange bed.

One explanation of the mechanism of the absorption of the HzSiFs and ofthe function of the HF band is as follows: The freshly alkaliregenerated exchange material is alkaline in reaction and in addition toabsorbing HzSiFc (see Equation 7) would tend to decompose HzSiFs just asany other alkali would do as per equation:

if I-IzSiFe were allowed to contact freshly alkali regenerated exchangematerial.

Now, if the HzSiFs solution (i. e. conditioned feedwater) is compelledto pass through a layer or zone of HF absorbed by exchange materialbefore it can contact freshly alkali regenerated exchange material, thereaction 7a is suppressed and the following reaction may be consideredto take place:

The exchange material when combined with HF (HF band) is no longeralkaline in reaction, and therefore does not tend to decompose theHzSiF'e. Hence this HF band may be considered as a barrier acting toprevent direct contact of the HzSiFs with freshly alkali regeneratedexchange material. The HF liberated according to Equation 71) is carriedforward through the bed by the water until it comes in contact withfreshly alkali regenerated exchange material, whereupon it will re-formY-H2F'2 as follows:

In this way the HF band. progresses through the bed and maintains itselfas a chemical barrier.

In addition to this function the HF band has an equalizing function inthe sense that it permits reasonable fluctuations of the HF-dosing ratewithout affecting the quality of the finished water. In other words, inspite of such possible fluctuations there is no leakage of silica intothe finished water. In case the HF-dosing rate has temporarily droppedbelow theoretical, this equalizing function of the HF-band can be saidto be due to the ability of the I-IF-band to react with silica asfollows:

While this equation appears to be contradictory to Equation 7a, itapparently takes place as long as there is sufficient excess of Y'H2F2present.

In case the HF-dosing rate has temporarily risen above theoretical,Equation 70 operates and stores HF in the band.

Substantially complete exhaustion of the anion exchange bed may beeffected in a two bed operation, but that is with two anion exchangebeds operating in series as indicated in Fig. 4.

This application is a division of parent applicaaqueous fluids, andseparating said solution from tion of Elliott J. Roberts and Walford W.Jukkola, said material.

B01131 NO. 588,388 flied April 14, 1945. MOT]: J. ROBERTS.

I claim: A process for the removal of silica from aque- 5 R ERENCESCITED ous fluids which mmprises adding to an aqueous The followingreferences are of record in the nuid containing silica an amount ofhydrofluoric m f this patent: acid suflicient to convert substantiallyall the silica present to fluosilicic acid, bring the solution UNITEDSTATES PATENTS so obtained into contact with an anion exchange 10 NumberName Date material active for the removal of anions from 2,155,318Liebknecht Apr. 18, 1939

